Ventricular arrhythmias are potentially lethal. In the instance of chaotic, non-coordinated muscle contraction, known as fibrillation, death can ensue within minutes after onset. To convert the fibrillation to an organized, slower cardiac rate, an electrical countershock is given. A high energy pulse of 400 joules or less is applied across the chest wall using an external defibrillator. However, such external defibrillators are located in hospitals and in rescue vehicles. Because death can ensue within ten minutes, medical assistance may arrive too late to resuscitate the patient.
For patients who have survived an episode of ventricular fibrillation, there is a high probability of reoccurrence. In addition, patients who have experienced sustained symptomatic ventricular tachycardia are at risk in that such arrhythmias may convert to fibrillation. It is these patients who benefit from an implantable cardioverter or defibrillator.
An implantable cardioverter or defibrillator must be capable of sensing ventricular cardiac electrical activity, of determining if the sensed electrical activity is ventricular tachycardia or fibrillation and of enabling a circuit which then delivers a high energy pulse to electrodes associated with the heart to perform cardioversion (a shock in synchronization with the cardiac cycle) or defibrillation.
In prior implantable defibrillators, a truncated exponential pulse (trapezoidal pulse) of 25 joules or more has been utilized. Such a trapezoidal pulse is produced by a few external defibrillators but, more commonly, a damped sine wave is used. External defibrillators require a higher energy source, of up to 400 joules, because of energy dissipation through the chest wall.
Schuder et al. in "Ultrahigh-Energy Hydrogen Thyratron/SCR Bidirectional Waveform Defibrillator," Med & Biol. Eng. & Comput. Vol. 20, pp. 419-424 (July, 1982), show a symmetrical bi-directional truncated exponential waveform having less than 10% droop, so as to approximate a rectangular waveform, and a schematic diagram of an apparatus for generating such a waveform. The bi-directional pulse as described by Schuder et al. is incorporated into a rather large external defibrillator. Such bi-directional pulses can defibrillate and do not appear to influence adversely the outcome of subsequent attempts to defibrillate. However, much of the energy stored in the capacitor is lost, because the charge on the capacitor at the end of each phase must be dumped when the voltage is still a large fraction of the initial charge voltage.
In an implantable defibrillator, it is necessary to conserve space so that the implantable unit is not large. Further, because battery energy is finite in an implantable unit, any increase in efficiency that can be attained by fuller utilization of the stored energy (as measured in joules) or reduction in defibrillation threshold is of critical importance. Such increase in efficiency extends the useful life of the implant, thus reducing the frequency of implant replacement and the cost associated with an inefficient device due to the cost of the device itself, and surgical and hospital expenses incurred as a result of replacement. While replacement is a rather simple procedure, the patient is nevertheless exposed to the risks inherent in any form of surgery. In addition, reduction in defibrillation threshold, while saving energy, also has the beneficial effect of reducing the discomfort experienced by the patient when the defibrillation shock is applied.